The present invention relates to memory devices and, more particularly, to nonvolatile memory devices using resistive material cells. Nonvolatile memories using resistive materials include phase-change random access memories (PRAMs), resistive RAMs (RRAMs), and magnetic RAMs (MRAMs). While dynamic RAMs (DRAMs) or flash memories typically store data using charge storage, nonvolatile memories having resistive materials typically store data by controlling a state of a phase-change material, such as chalcogenide alloy (in the case of PRAMs), the resistance of a variable resistive material (in the case of RRAMs), or the resistance of a magnetic tunnel junction (MTJ) thin film according to the magnetization state of a ferromagnetic material (in the case of MRAMs).
In particular, the phase-change material of a typical PRAM becomes crystalline or amorphous as it is cooled after being heated. The phase-change material typically has a low resistance in the crystalline state and has a high resistance in the amorphous state. Therefore, the crystalline state may be defined as “set” (or “0”), and the amorphous state may be defined as “reset” (or “1”). As nonvolatile memories have greater storage capacity and become more highly integrated, bitlines of some nonvolatile memories are being implemented using a hierarchical bitline structure having global bitlines and local bitlines. In particular, the global bitlines may be divided into read global bitlines for reading data from nonvolatile memory cells and write global bitlines for writing data to the nonvolatile memory cells. Using such structures, the storage capacity and integration density of the nonvolatile memories may be further increased. Recently, methods of accurately reading data from each nonvolatile memory cell of nonvolatile memories structured as described above are being researched and developed.